The goal is to achieve FEL lasing at photon energy of 42 keV. We assume the use of the undulator, which has already been designed for MaRIE [1]}. That requires the electron beam to reach the energy of 12 GeV and peak current of 3 kA at the undulator entrance. Most of the acceleration is envisioned to happen in plasma. The uncertainty in the beam's quality and its high energy after the LWFA stage does not allow for temporal compression of the beam prior to undulator. That implies that the electron beam needs to be fully compressed prior to injection into the LWFA. This setup allows to keep the induced energy spread at the minimum level since the bunch inside the plasma wake is short. Reaching beam current of 3 kA is a challenging task for high brightness accelerators. The MaRIE linac design was able to reach high current at energies as low as 1 GeV [2]}. It is extremely difficult to design an accelerator reaching full compression at significantly lower energy. As a result, the RF accelerator must provide acceleration for high brightness beams to the energy of \(\sim \) 1 GeV with the peak current of \(\sim \)  3 kA. The conventional approach to designing such systems is to implement 2-3 compressors at intermediate energies [3]}, [4]}, [5]}, [6]}, [7]}, [2]}. In this paper we assume that a low energy linac section designed for MaRIE can be used to deliver the beam with required parameters.
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